Thin film magnetic recording disks and disk drives are conventionally employed for storing large amounts of data in magnetizable form. Typically, one or more disks are rotated on a central axis in combination with data transducer heads. In operation, a typical contact start/stop (CSS) method commences when the head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk due to dynamic pressure effects caused by air flow generated between the sliding surface of the head and the disk. During reading and recording operations, the transducer head is maintained at a controlled distance from the recording surface, supported on a bearing of air as the disk rotates, such that the head can be freely moved in both the circumferential and radial directions allowing data to be recorded on and retrieved from the surface of the disk at a desired position. Upon terminating operation of the disk drive, the rotational speed of the disk decreases and the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Thus, the transducer head contacts the recording surface whenever the disk is stationary, accelerated from the stop and during deceleration just prior to completely stopping. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic operation consisting of stopping, sliding against the surface of the disk, floating in the air, sliding against the surface of the disk and stopping.
It is considered desirable during reading and recording operations to maintain each transducer head as close to its associated recording surface as possible, i.e., to minimize the flying height of the head. Thus, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head, thereby permitting the head and the disk to be positioned in close proximity with an attendant increase in predictability and consistent behavior of the air bearing supporting the head. However, if the head surface and the recording surface are too flat, the precision match of these surfaces gives rise to excessive stiction and friction during the start up and stopping phases, thereby causing wear to the head and recording surfaces eventually leading to what is referred to as a "head crash." Thus, there are competing goals of reduced head/disk friction and minimum transducer flying height.
Conventional practices for addressing these apparent competing objectives involve providing a magnetic disk with a roughened recording surface to reduce the head/disk friction by techniques generally referred to as "texturing." Conventional texturing techniques involve polishing the surface of a disk substrate to provide a texture thereon prior to subsequent deposition of layers, such as an underlayer, a magnetic layer, a protective overcoat, and a lubricant topcoat, wherein the textured surface on the substrate is intended to be substantially replicated in the subsequently deposited layers.
A typical magnetic recording medium is depicted in FIG. 1 and comprises a substrate 10, typically an aluminum (Al)-base alloy, such as an aluminum-magnesium (Al--Mg) alloy, plated with a layer of amorphous nickel-phosphorous (NiP). There are typically sequentially deposited on substrate 10 a chromium (Cr) or Cr-alloy underlayer 11, a magnetic layer 12 which is usually a cobalt (Co) -base alloy, a protective overcoat 13 which usually comprises carbon, and a lubricant topcoat 14. Cr or Cr-alloy underlayer 11, Co-base alloy magnetic layer 12 and protective carbon overcoat 13 are typically deposited by sputtering techniques. A conventional Al-alloy substrate is provided with a NiP plating primarily to increase the hardness of the Al substrate, serving as a suitable surface for polishing to provide the requisite surface roughness or texture, which is intended to be substantially replicated on the disk surface.
The escalating requirements for high areal recording density impose increasingly greater requirements on thin film magnetic media in terms of coercivity, stiction, squareness, low medium noise and narrow track recording performance. In addition, increasingly high density and large-capacity magnetic disks require increasingly smaller flying heights, i.e., the distance by which the head floats above the surface of the disk in the CSS drive. The requirement to further reduce the flying height of the transducer head renders it particularly difficult to satisfy the requirements for controlled texturing to avoid head crash.
Conventional techniques for providing a disk substrate with a textured surface comprise a mechanical operation, such as polishing. See, for example, Nakamura et al., U.S. Pat. No. 5,202,810. Conventional mechanical texturing techniques are attendant with numerous disadvantages. For example, it is extremely difficult to provide a clean textured surface due to debris formed by mechanical abrasions. Moreover, the surface inevitably becomes scratched during mechanical operations, which contributes to poor glide characteristics and higher defects. In addition, various desirable substrates are difficult to process by mechanical texturing. This undesirably limiting facet of mechanical texturing, virtually excludes the use of many materials for use as substrates.
An alternative texturing technique to mechanical texturing comprises the use of a laser light beam focused on an upper surface of a non-magnetic substrate. See, for example, Ranjan et al., U.S. Pat. No. 5,062,021, wherein the disclosed method comprises polishing an NiP plated Al substrate to a specular finish, and then rotating the disk while directing pulsed laser energy over a limited portion of the radius, to provide a textured landing zone leaving the data zone specular. The landing zone comprises a plurality of individual laser spots characterized by a central depression surrounded by a substantially circular raised rim.
Another laser texturing technique is reported by Baumgart et al. "A New Laser Texturing Technique for High Performance Magnetic Disk Drives," IEEE Transactions on Magnetics, Vol. 31, No. 6, pp. 2946-2951, November 1995. The laser texturing technique disclosed by Baumgart et al. employs a single focusing lens, and the shape of the resulting protrusions are shown to be altered by adjusting the pulse energy. At low pulse energies, the bump or protrusion shape comprises a central depression and a surrounding rim, similar to that reported by Ranjan et al. As the pulse energy is increased, the bottom of the depression flattens into a rounded, smooth, central dome resembling a "sombrero." At higher powers, the central dome broadens and decreases in height to eventually become equal to or lower than the rim.
In copending application Ser. No. 08/666,374 filed on Jun. 27, 1996, a laser texturing technique is disclosed employing a multiple lens focusing system for improved control of the resulting topographical texture. In copending application Ser. No. 08/666,374 filed on Jun. 27, 1996, a laser texturing technique is disclosed wherein a pulsed, focused laser light beam is passed through a crystal material to control the spacing between resulting protrusions.
Conventional laser texturing techniques have previously been applied to metal-containing substrates or substrates having a metal-containing surface, such as Ni--P plated Al or Al-base alloys. Such substrates, however, exhibit a tendency toward corrosion and are relatively fragile, thereby limiting their utility so that they are not particularly desirable for use in mobile computer data storage applications, such as laptop computers. Glass and glass-ceramic substrates exhibit superior resistance to shock than Ni--P coated Al or Al-alloy substrates. Accordingly, glass and glass-ceramic substrates are desirable candidates for use in mobile computer data storage applications. However, it is extremely difficult to provide an adequate texture on a glass or a glass-ceramic substrate, particularly in view of the escalating requirements for high areal recording density.
Conventional practices for texturing a glass or glass-ceramic substrate comprise heat treatment. Goto et al., U.S. Pat. No. 5,391,522, discloses a glass ceramic substrate suitable for use in a magnetic recording medium. A textured surface is provided by heat treatment, during which the recrystallization temperature is maintained for about 1 to about 5 hours to generate secondary crystal grains forming the surface texture characterized by irregular protrusions with surrounding valleys extending into substrate.
Hoover et al., U.S. Pat. No. 5,273,834 discloses the use of alternate substrates, such as glass-ceramic substrates. The substrate material is provided with ions for absorbing radiation in the near infrared portion of the spectrum, thereby rendering the material capable of attaining elevated temperatures during film deposition.
The use of heat treatment to form a textured surface on alternate substrates, such as glass or glass-ceramic substrates, is undesirably slow and inefficient in terms of energy consumption. Significantly, it is extremely difficult to exercise control over the size and shape of the secondary crystal grains due to inherent limitations in controlling temperature uniformity. Accordingly, it is virtually impossible to provide a glass or glass-ceramic substrate with a controlled textured landing zone for optimizing flying height and maximizing data zone recording density. Moreover, the resulting texture comprises irregularly shaped protrusions with surrounding valleys extending into the substrate, thereby creating undesirable stress profiles during subsequent deposition of layers by sputtering at elevated temperatures. Such undesirable stress profiles render it extremely difficult to accurately replicate the texture in subsequently deposited layers.
In copending PCT application Ser. No. 96,06830, filed May 9, 1996, a method is disclosed for laser texturing a glass or glass-ceramic substrate employing a laser light beam derived from a CO.sub.2 laser source. The textured glass or glass-ceramic substrate surface comprises a plurality of protrusions which extend above the substrate surface, without surrounding valleys extending substantially into the substrate as is characteristic of a laser textured metallic substrate. The effect of laser parameters, such as pulse width, spot size and pulse energy, and substrate composition on the protrusion or bump height of a laser textured glass or glass-ceramic substrate is reported by Kuo et al., "Laser Zone Texturing on Glass and Glass-Ceramic Substrates," presented at The Magnetic Recording Conference (TMRC), Santa Clara, Calif., Aug. 19-21, 1996. In copending application Ser. No. 08/796,830 filed on Feb. 7, 1997, there is disclosed a method of manufacturing a magnetic recording medium, which method comprises laser texturing a glass or glass-ceramic substrate, and controlling the height of the protrusions by controlling the quench rate during resolidification of the laser formed protrusions, as by heating the substrate during resolidification.
There remains a need for a magnetic recording medium comprising a glass or glass-ceramic substrate having an accurately controlled texture, and for a method of laser texturing a glass-ceramic substrate, wherein the height of the protrusions extending above the substrate surface is controlled and the sensitivity of the protrusion height to pulse energy is reduced.